Power converters, for example, AC/DC converters, are usually equipped with power factor correction means or circuits. An intermediate storage capacitor is typically used to provide the necessary power factor correction or adjustment. However, the intermediate storage capacitor for power factor correction is usually subject to a high voltage stress as the voltage of the intermediate storage capacitor is usually left uncontrolled and can vary widely with respect to the line voltage and the load current. Consequently, the storage capacitor voltage can be substantially higher than the peak line voltage.
For example, while the ordinary line input voltage ranges from 90 to 265 Vrms, the voltage across the intermediate storage capacitor can vary between 140V to 2500V. If the DC/DC regulator stage operates in the continuous conduction mode (“CCM”) and at a decreasing load, the storage capacitor voltage can go up even higher due to power imbalance between the input and output.
As a result, a bulkier storage capacitor with a higher voltage rating as well as other high-voltage-rating devices (such as power switches and diodes) which inevitably lead to an increase of the size and the total costs will have to be used.
Furthermore, as single-stage power-factor-corrected converters (SSPFC) aiming at reducing the cost and simplifying the power stages and control of the converter have been developed by integrating a power factor correction (PFC) circuit with a DC/DC regulator circuit and is becoming more useful, there is therefore an urging need to devise improved power factor corrected power converters so that the demand on the voltage rating of the intermediate storage capacitor can be lessened so that a less bulky storage capacitor with a lower voltage rating can be used.
In order to alleviate the above problems, various schemes and methodologies such as the use of variable frequency control, bus voltage feedback control and series-charging-parallel-discharging techniques have been reported. In addition, it has been suggested to alleviate the problems by inserting a direct power transfer path to the input stage of a converter to raise conversion efficiency and to lower the voltage stress on the storage capacitor. However, the large storage capacitor voltage swing due to line voltage variation remains a largely unresolved problem. In particular, the voltage across the storage capacitor of the known power-factor-corrected power converters always exceed the peak line input voltage due to the presence of a boost converter in such topologies which inevitably steps up the voltage across the storage capacitor. Garcia et al in “AC/DC Converters with tight output voltage regulation and with a single control loop,” in IEEE Power Electronics Specialists Conf., 1999, pp. 1098–1104, and Lazaro et al, in “New family of single-stage PFC converters with series inductance interval,” in IEEE Power Electronics Specialists Conf, 200, pp. 1357–1362 attempted to reduce the storage capacitor voltage below the peak line voltage by using flyback-buckboost and flyback-boost converters respectively. However, such converters require two switches and are less attractive for low-power applications.